Add bestseller rankings challenge

src/data/courses/sql-mastery/chapters/subqueries-and-ctes/lessons/bestseller-rankings.md

@@ -0,0 +1,91 @@
+---
+title: Bestseller Rankings
+description: Practice using CTEs to rank books by their sales
+order: 143
+type: challenge
+setup: |
+  ```sql
+  CREATE TABLE book (
+      id INT PRIMARY KEY,
+      title VARCHAR(100),
+      category VARCHAR(50),
+      price DECIMAL(10,2),
+      sales_count INT
+  );
+
+  INSERT INTO book (id, title, category, price, sales_count) VALUES
+      (1, 'SQL Basics', 'Programming', 29.99, 150),
+      (2, 'Advanced SQL', 'Programming', 49.99, 100),
+      (3, 'Data Science', 'Data Analysis', 39.99, 120),
+      (4, 'Web Development', 'Programming', 34.99, 80),
+      (5, 'Statistics 101', 'Data Analysis', 24.99, 200),
+      (6, 'Machine Learning', 'Data Analysis', 44.99, 90);
+  ```
+---
+
+The bookstore wants to create a bestseller list that shows each book's sales and its percentage contribution to total sales across all books.
+
+Given the following data in table `book`:
+
+| id  | title            | category      | price | sales_count |
+| --- | ---------------- | ------------- | ----- | ----------- |
+| 1   | SQL Basics       | Programming   | 29.99 | 150         |
+| 2   | Advanced SQL     | Programming   | 49.99 | 100         |
+| 3   | Data Science     | Data Analysis | 39.99 | 120         |
+| 4   | Web Development  | Programming   | 34.99 | 80          |
+| 5   | Statistics 101   | Data Analysis | 24.99 | 200         |
+| 6   | Machine Learning | Data Analysis | 44.99 | 90          |
+
+Write a query using a CTE that shows:
+
+- Book title
+- Sales count
+- Percentage of total sales (rounded to 1 decimal place)
+
+Sort the results by sales count in descending order.
+
+## Expected Output
+
+| title            | sales_count | sales_percentage |
+| ---------------- | ----------- | ---------------- |
+| Statistics 101   | 200         | 27.0             |
+| SQL Basics       | 150         | 20.3             |
+| Data Science     | 120         | 16.2             |
+| Advanced SQL     | 100         | 13.5             |
+| Machine Learning | 90          | 12.2             |
+| Web Development  | 80          | 10.8             |
+
+> **Note:** Each book's percentage is calculated as (book_sales / total_sales \* 100)
+
+## Solution
+
+```sql
+WITH sales_total AS (
+    SELECT SUM(sales_count) as total_sales
+    FROM book
+)
+SELECT
+    title,
+    sales_count,
+    ROUND(
+        (CAST(sales_count AS DECIMAL) / total_sales * 100),
+        1
+    ) as sales_percentage
+FROM book
+CROSS JOIN sales_total
+ORDER BY sales_count DESC;
+```
+
+This challenge requires you to:
+
+1. Create a CTE to calculate the total sales across all books
+2. Use the CTE to calculate each book's percentage of total sales
+3. Format the results with proper rounding
+
+The solution demonstrates:
+
+- Basic CTE usage to store an intermediate calculation
+- Cross join between the CTE and main table
+- Percentage calculation with proper decimal handling
+
+---